METALS AND MATERIAl_S, Vol. 5, No. 6 (1999), pp. 545-549

Development of Lead (Pb)-free Interconnection Materials for Microelectronics Sung K. Kang IBM T. J. Watson Research Center EO. Box 218, Yorktown Heights, NY 10598, USA Although the amount of Pb or Pb-containing solders used in microelectronics counts only a small fraction of the total Pb usage, the demand for manufacturing "green" products has promoted active research and developmentfor the Pb-free interconnectionmaterials in the recent years. As a result, several candidate materials are now available in two categories: Pb-free solders and electrically conducting adhesives. In this paper, the new development activities of both materials are briefly reviewed in terms of their promising properties, applications, and concerns.

Key words : Pb-free solders, electrically conducting adhesives, electronic packaging

1. I N T R O D U C T I O N Most electrical conductors used in electronic devices are made of metals, such as copper, aluminum, gold, silver, lead/tin alloys, molybdenum and others. Solder connection technology using lead/tin alloys plays a key role in various levels of electronic packaging [ 1,2], such as flip-chip connection (or C4), solder-ball connection in ball-grid arrays (BGA), and IC package assembly to a printed circuit board (PCB). Solder joints produced in the electronic packages serve critically as electrical interconnections as well as mechanical/physical connections. When either of the functions is not achieved, the solder joint is considered to have failed, which can often threaten a shutdown of the whole electronic system. Among various lead/tin alloys, the lead-tin eutectic solder, 63%Sn-37%Pb, is most widely used in electronic applications because of its lowest melting point, 183~ When IC packages, resistors, capacitors, or connectors are attached to a printed circuit board, the Pb-Sn eutectic solder is commonly used. In these applications, there are two solder connection technologies employed for mass production: plated-through-hole (PTH) and surface mount technology (SMT) soldering. The basic difference between the two technologies originates from the difference in the PCB design and its interconnection scheme. In SMT sol-

dering, microelectronic packages are directly attached to the surface of a PCB, while in Find soldering, the package leads are inserted into PTHs and solder joints are made within the PTHs of a PCB. A major advantage of SMT is a high packaging density, which is realized by eliminating most PTHs in the PCB as well as by utilizing both surfaces of the PCB to accommodate components. In addition, SMT packages have a finer lead pitch and a smaller package size compared to the conventional FFH packages. Hence, SMT has contributed significantly in reducing the size of electronic packages and thereby the volume of the overall system. Recent advances in microelectronic devices demand a very fine pitch connection between electronic packages and a PCB. The current solder paste technology used in SMT can not handle this very fine pitch interconnection due to the possible soldering defects such as bridging or solder balling [3]. Another technical limitation of using the Pb-Sn eutectic solder is its high reflow temperature, approximately 215~ This temperature is already higher than the glass transition temperature of the epoxy resin used in most polymeric PCB materials [4]. Thermal exposure at this reflow temperature produces significant thermal strains in a PCB after soldering, especially in the direction perpendicular to the surface of a PCB, because no structural reinforcement is made in that direction. Thereby, the residual thermal strains in an assembled

This articlebased on a presentationmade in the symposium"The 1999 KSEAMaterialsSymposiumin Honorof ProfessorSang Joo Kim's 70th Anniversary",held at UCLA,Los Angeles,CA, USA, August 12-14, 1999 under the auspices of Korea-US AmericanScientistsand Engineers Association.

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PCB could significantly degrade the reliability of an electronic system. Another serious concern regarding the usage of lead (Pb) containing solders is an environmental issue [5], a trend already experienced in other industries and had led to the elimination of lead from gasoline and paints. In the electronic industry, two different materials have been investigated for the possibility of substituting the Pb-containing solders; Pb-free solder alloys [6-8], and electrically conducting adhesive materials [9-16]. The present paper discusses the recent development and applications of both materials.

2. LEAD(Pb)-FREE SOLDERS An acceptable Pb-free solder must satisfy both process requirements and reliability objectives. Ideally, it should be suitable for mass production applications of both SMT and PTH soldering. Furthermore, the process technologies currently in use for Pb-Sn eutectic or other Pb-containing solders must be adaptable to the new solder without major changes, or significant capital investments. The wettability of the new solder should be better or equivalent to that of the Pb-Sn solder, and accordingly a better or equivalent defect rate in the assembly line should be achievable. It should be workable with water-washable or no-clean fluxes. The melting point of the new solder is desired to be lower than those of the current solders in order to reduce the magnitude of thermal stresses or thermal shock experienced during soldering. The new solder must be able to produce solder joints with acceptable joint strength, and at the same time must withstand thermal fatigue over the projected operating life of the assembly and meet other reliability requirements, such as corrosion or oxidation. Lastly, the material cost of the new solder should not be so high that it may overwhelm the assembly cost. With the above guidelines in mind, several Pb-free solders have been reinvestigated or newly developed recently [5-8]. Some

of the examples include 42%Sn-58%Bi, Sn-3.5%Ag, Sn3.5%Ag-4.8%Bi, Sn-3.5%Ag-0.7%Cu, Sn-0.7%Cu, Sn5%Sb, and others. Table 1 summarizes a few salient features of these Pb-free solder alloys in terms of their favorable properties, applications and concerns. Some of the alloy compositions are already available commercially in the form of solder pastes to be used for the reflow soldering in SMT. However, as stated in the final report of the National Center for Manufacturing Sciences [8], no drop-in replacement for the eutectic Pb-Sn solder has been found yet in general-purpose applications. For several promising solders such as listed in Table 1, there are very limited amounts of product reliability data available in comparison to that of the Pb-containing solders. Therefore, further investigations on solderability and product reliability are required to replace the Pb-containing solders with confidence in addition to considering other important issues such as economic or ecological/environmental ones.

3. CONDUCTING ADHESIVE MATERIALS An electrically conducting adhesive material is made of metallic filler particles loaded in the matrix of a polymer material. The polymer matrix can be either thermoplastic or thermoset or their mixture. Silver-particle filled epoxy is the most common example of the electrically conducting adhesive materials in use. The silver particles usually in the shape of flakes provide electrical conduction by percolation mechanism, while the epoxy matrix provides adhesive bond between the components and a substrate. The silver-filled epoxy material has been long used in the electronic applications as a die-bonding material [12], where its good thermal conduction rather than electrical conduction property is utilized. Recently, this material has been further developed for applications requiring high electroconduction and fine pitch connection [9-18]. The silver-filled epoxy material has still several limitations

Table I. Pb-free solder alloys for microelectronic applications Composition Melting Favorable properties Applications (wt.%) Temp (~ 58Bi-42Sn 139 fatiguelife creep resistance, low temp process, joint strength PTH soldering Sn-3.5Ag 221 fatiguelife wettability,joint strength solderbump, flip chip joining Sn-3.5Ag-4.8Bi 208-215 wettability,fatigue life SMT Sn-3.5Ag-0.7Cu 217 less dissolution, fatigue life SMT, wave soldering Sn-0.7Cu 227 low cost wave soldering Sn-5Sb 232-240 creepresistance, joint strength, pin joining, hermatic seal, fatigue life chip joining

Concerns poor wettability, low mp (BiPbSn) by-product of Pb Cu dissolution, intermetallics fillet lift in PTH, low mp (BiPbSn) OSP wetting, voiding on finish poor wetting poor wetting, Cu dissolution intermetallics

Development of Lead (Pb)-free Interconnection Materials for Microelectronics for the applications, such as low electrical conductivity, increase in contact resistance during thermal exposure, low joint strength, silver migration, difficulty in rework, and others. Since the silver-filled epoxy material is electrically conductive in all directions, it is classified as an "isotropic" material. There is another class of electrically conducting material, which provides electrical conduction only in one direction. This class of the material is called as an "anisotropic" conducting adhesive or film [19-22]. Since the filler loading level is so low in an anisotropic conducting adhesive, it becomes conductive only when it is compressed between two conducting pads. This process normally requires heat and pressure. The major application of the anisotropic conductive film has been for interconnecting a liquid crystal display panel to its electronic devices. The conducting particles in use include deformable ones such as solder balls, or plastic balls coated with nickel and gold [21], or hard particles such as nickel [22]. In the following, the discussion is mainly focused on the isotropic materials. In order to overcome the limitations of the silver-filled epoxy materials, several new formulations have been developed recently for various applications. To meet the reworkability requirement in high performance applications, such as in a multichip module (MCM), a reworkable thermoplastic resin has been incorporated with a proper solvent and silver particles [15]. Solvent removal from the thermoplastic resin has been carefully controlled during the reflow cycle. Another reworkable thermoplastic conductive paste has also been reported for a fine-pitch lipchip application [11]. Here, by loading up the fine silver particles more than 40% in volume, a high electrical conductivity better than that of the Pb-Sn eutectic solder has been achieved. A new class of conductive adhesive materials has been developed by replacing silver particles with other conducting particles, such as solder particles [23], a mixture of solder and copper [13,25], tin-coated copper [16,26], silver-coated copper [24], and others [27]. A solder/polymer composite paste material has been developed by mixing solder powder particles, thermoplastic polymer resin in a volatile solvent, and a fluxing agent [23]. Upon reflow, an oxide-free, partially coalesced solder connection is obtained, which is polymer strengthened and reworkable at a low reflow temperature or in the presence of polymer solvent. A hybrid of solder and conductive adhesive joining technologies has been developed to exploit the advantages of both [13]. This new conductive adhesive is a mixture of a solder powder, a metal powder of high melting point such as copper, a fuxing agent, a polymer resin and others.

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Here, the electrical connection is established through transient liquid phase sintering (TLPS) among metal and solder powder as well as to the conducting pads. A promising result has been reported with the SMT joints made of the TLPS conductive paste in terms of electrical conductivity, impact strength, and reliability, which are substantially better than those of the conventional conductive adhesives [25]. Another improvement has been reported with the high conductivity Pb-free conducting materials made of a conducting filler powder coated with a low melting point metal, a thermoplastic polymer resin, and other minor organic additives [26,27]. Similar to the TLPS conductive paste, these high conductivity Pb-free conducting materials do also provide metallurgical bonding between adjacent filler particles, and between the filler particles and the contact pads to be joined, in addition to the adhesive bonding from the polymer matrix. In the following, development of the high conductivity Pb-free conducting materials is further described for two different applications; one for high temperature applications with ceramic substrates, and another for low temperature with organic substrates.

3.1. High temperature conducting adhesives This new electrically conducting adhesive material consists of a conducting filler powder coated with a low melting point metal, a thermoplastic polymer resin, and other minor organic additives. A conducting filler powder is selected from the group consisting of Au, Cu, Ag, AI, Pd and Pt. The filler particles are coated with low melting point, nontoxic metals which can be fused to achieve metallurgical bonding between adjacent filler particles, and between the particles and the contact surfaces that are joined using the adhesive material. The coating layer is selected from the group of fusible metals, such as Bi, In, Sn, Sb, Zn and their alloys. The polymeric material is selected from the group consisting of polyimide, siloxane, polyimide-siloxane, polyester, phenoxy, styrene allyl alcohol, and others. Since the present conducting adhesive is primarily based on particle-particle and particle-pad metallurgical bonds, the critical volume fraction of the filler material required to achieve an acceptable conductivity level is much less than the conventional silver-epoxy adhesive, which relies on physical contact and percolation mechanism among the filler particles. An example of high temperature conductive adhesive is made of tin-coated copper powder filler, a thermoplastic polyimide-siloxane resin, and other additives. This material is a good candidate in replacing the high temperature solder joints, such as controlled collapse chip connections (C4) and solder

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Table 2. Electrical and mechanicalproperties of model joints made of new conductiveadhesive materials Filler

Loading (wt. %)

Resin

Bonding ("C, p s i )

Electrical (10-2~)

Mechanical (lb)

Ag 80-90 Epoxy Pb/Sn >90 Solder Sn/Cu high PS' Sn/Cu medium PS Sn/Cu low PS BiSn/Cu medium LCP~ BiSn/Cu medium LCP BiSn/Cu medium LCP "PS=polyimide-siloxane,LCP=low cost polymer. *Average value of 10 measurementsand their stmadarddeviation. The apparentjoint area is 1.25 mm2.

160, 25 215, 25 250, 25 250, 25 250, 25 188, 25 188, 25 188, 25

7.3 (2.2)* 4.7 (1.0) 2.6 (0.9) 6.0 (1.2) 8.6 (4.1) l l.0 (4.0) 8.0 (2.0) 8.0 (1.0)

6.6 (0.4)* 10.2 (0.1) 5.6 (0.8) 7.2 (0.9) 9.5 (0.5) 8.3 (0.8) 10.0 (0.6) 9.0 (0.8)

ball connection (SBC) to a ceramic substrate. A typical joint microstructure of the Sn/Cu conducting adhesive consists of two distinct regions; one where the conducting particles fused to the copper substrates as well as to other conducting filler particles to form a metallurgical joint, and another region where the polymer resin forms adhesive bonding. The metallurgical bond among the conducting particles provides better electrical conduction and higher mechanical strength of the joint. Table 2 summarizes the electrical resistance values and mechanical strength of the model joints formed with this high temperature Sn/Cu conducting adhesive material. The electrical resistance value of the Pb/Sn solder joint serves as a reference for comparison. The resistance of the silverepoxy joint is about 50% higher than that of the Pb/Sn. The joint resistance of the new Sn/Cu adhesive with a high filler loading shows even a lower resistance than that of the Pb/Sn solder joint. The resistance of the Sn/Cu adhesive with a medium loading is close to that of the Pb/Sn. Average shear strength of the model joints made with the present Pb-free, Sn/Cu conducting adhesive material is compared with those of other joints in Table 2. As expected, the Pb/Sn solder joint shows the highest joint strength. The joint shear strength of the Sn/Cu adhesive materials varies according to the level of the filler loading, but in an opposite way, as the electrical resistance of the joints does. The joint strength of the Sn/Cu adhesive materials decreases as the level of the filler loading increases. In general, the Pb-free, Sn/Cu adhesive material demonstrates a better joint strength than the silver-epoxy material, and for a low loading formulation it shows a joint strength very close to that of the Pb/Sn solder joint. A few reliability tests conducted so far with the Sn/Cu adhesive have shown promising results for the applications of direct chip attach and discrete device attachment to a ceramic substrate.

3.2. Low temperature conducting adhesives Low temperature adhesive materials are made of conducting copper powder coated with a thin layer of low melting point, Pb-free metals selected from Sn, In, Bi, Sb, Zn and their alloys. The conducting particles are mixed with an environmentally-safe fluxing agent, and dispersed in the matrix of thermoplastic or thermosetting polymers. Here, we present one example of low temperature adhesive materials containing conducting copper powder coated with a thin layer of BiSn alloys whose melting points are below 200~ This conducting filler particles of a few micron in diameter are mixed with a thermoplastic polymer and a fluxing agent to formulate a low temperature conducting adhesive. A typical bonding condition for the low temperature conductive adhesive material is listed in Table 2 with the measured electrical and mechanical properties of the model joints. The reduction of the bonding temperature from 250~ to 188~ is achieved by replacing tin-coated copper powder with the BiSn-coated copper. This low bond temperature is even lower than the reflow temperature of solder paste, 215~ commonly practiced in SMT soldering. The average values of contact resistance of the BiSn/Cu samples exhibit slightly higher than those of the Sn/Cu ones. But the joint strength of the BiSn/Cu Table 3. Properties of electricallyconductiveadhesive materials Properties

Ag-Epoxy

Pb/Sn Solder

Sn/Cu in PSt

BiSn/Cu in LCP"

Resistivity 80-300 30 17-55 50-70 (~f~-cm) Contact R 1.95-7.3 0.73 0.41-1.34 1.2-1.8 (pXq-cm~) Strength 1,500 4,080 2,200-3,800 3,300-4,000 (psi) *The property values cover three different formulations:low, medium and high loading of the filler material. "The property values cover only for the medium loading.

Development of Lead (Pb)-free lnterconnection Materialsfor Microelectronics

samples demonstrate higher values than those of the Srg Cu. Table 3 compares several salient properties of the low temperature conducting adhesive, BiSn/Cu, with those of other conducting materials, such as silver-filled epoxy, tinlead solder paste of 63%Sn-37%Pb eutectic composition, and tin-coated copper conducting adhesive denoted by Sn/ Cu. The electric~ and mechanical properties of the new electrically conducting adhesives, Sn/Cu and BiSn/Cu, are significantly better than those of the commercial silverfilled epoxy, and are also comparable to those of the solder joint. In summary, the status of the Pb-free interconnection materials for microelectronics has been briefly reviewed by discussing two categories of the materials, namely, Pbfree solder alloys and electrically conducting adhesive materials. Due to the recent efforts in research and development, several promising materials have been identified in both categories of the materials. However, none of the candidate materials can replace directly or in general the existing Pb-Sn solders with the same degree of confidence in product reliability yet. Further research and development activities are strongly suggested in this area to facilitate the replacement of the Pb-containing materials without sacrificing the well-established reliability record as well as the economical advantages associated with the Pb-containing materials.

REFERENCE 1. N. Koopman, T. Reiley, P. Totta, Microelectronics Packaging Handbook (eds., R. Tummala, E. Rymaszewski), p. 361, Van Reinhold (1989). 2. T. Reiley, D. Shin, Microelectronics Packaging Handbook (eds., R. Tummala, E. Rymaszewski), p. 779, Van Reinhold (1989). 3. J. Hwang, Solder Joint Reliability (ed., J. Lau), p. 38, Van Reinhold (1991). 4.. Schmit, B. Appelt, J. Gotro, Principles of Electronic Packaging (eds., D. Seraphim,R. Lasky, C. Li), p. 335, McGraw-Hill Book Co. (1989). 5. B. R. Allenby, J. Ciccarclli, I. Artaki, J. Fisher, D. Schoe-

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nthaler, T. Carroll, D. Dahringer, Y. Degani, R. Freund, T. Gradel, A. Lyons, J. Plewes, C. Gherman, H. Solomon, C. Melton, C. Munie, N. Socolowski, Proc. Surface Mount Int., vol. 1, p. 1, San Jose, CA, USA (1992). 6. S. K. Kang and A. Sarkhel, J. Elec. Materials 23, 701 (1994). 7. H. Steen, Elec. Pack. Prod. 12, 32 (1994) 8. Lead-Free Solder Project Final Report, National Center for Manufacturing Sciences, Ann Arbor, MI, USA (1997). 9. G. P. Ngyuen, J. Williams, F. Gibson, Circuit Assembly 1, 36 (1993). 10. K. Gilleo, Circuit Assembly 1, 52 (1994); 2, 50 (1994). 11. R. F. Saraf, J. M. Roldan, Proc. 45'h Elec. Comp. Tech. Conf., p. 1051, Las Vegas, USA (1995). 12. K. Gilleo, Elec. Pack. Prod. 2, 109 (1994). 13. C. Gallagher, G. Matijasevic, M. Capote, Prod. Surf. Mount Tech. Int. Conf., p. 568, San Jose, CA, USA (1995). 14. S. K. Kang, S. Pumshothaman, Proc. IEEE Int. Sym. on Electronics & The Environment, p. 177, Orlando, FL, USA (1995). 15. R. Dietz and D. Peck, Microelectron. Int. 1, 52 (1996). 16. S. K. Kang, R. Rai and S. Purushothaman, Proc. 46th ECTC, p. 565, Orlando, FL, USA (1996). 17. D. Klosterman, L. Li, J. Morris, Proc. 46th ECTC, p. 571, Orlando, FL, USA (1996). 18. S. Kotthaus, B. Gunther, R. Haug, H. Schafer, IEEE Trans. CPMT, Part A 3, 15 (1997). 19. R. Reinke, Proc. 41st Elec. Comp. Tech. Conf., p. 355 (1991). 20. S. Asai, U. Saruta, M. Tobita, M. Takano, Y. Miyashita, J. Appl. Polymer Sci. 5, 769 (1995). 21. K. Adachi, Solid St. Tech. 1, 63 (1993). 22. M. J. Yim, K. W. Paik, 48th ECTC, vol. 5, p. 1036, Seattle, WA, USA (1998). 23. W. S. Huang, I. Khandros, R. Saraf, L. Shi, US Pat, No.5,062,896, Nov. 5 (1991). 24. Y. Iwasa, Elec. Pack. Prod. 11, 93 (1997). 25. C. Gallagher, G. Matijasevic and J. E Maguire, Proc. 47th ECTC, p. 554, San Jose, CA, USA (1997). 26. S. K. Kang, R. S. Rai, S. Purushothaman, IEEE Trans. CPMT, Part A 3, 18 (1998). 27. S. K. Kang and S. Pumshothaman, Proc. 48th ECTC, p. 1031, Seattle, WA, USA (1998).